Biaxially oriented polypropylene dielectric film, modified polypropylene material and use thereof

Abstract

The invention belongs to the field of polymers, and relates to biaxially oriented polypropylene dielectric film, a modified polypropylene material and use thereof. The preparation raw materials of the biaxially oriented polypropylene dielectric film comprise a modified polypropylene grafted with an alkenyl-containing functional monomer. The modified polypropylene grafted with an alkenyl-containing functional monomer comprises a structural unit derived from polypropylene as a matrix phase and a structural unit derived from an alkenyl-containing functional monomer as a dispersion phase. The ash content of the modified polypropylene grafted with an alkenyl-containing functional monomer is less than 50 ppm. In the modified polypropylene grafted with an alkenyl-containing functional monomer, the mass ratio of the structure unit which is derived from an alkenyl-containing functional monomer and in a grafted state to the structure unit which is derived from an alkenyl-containing functional monomer and in a self-polymerization state is greater than or equal to 1.0. D50 of the dispersion phase is less than 450 nm. The biaxially oriented polypropylene dielectric film of the present invention can keep good dielectric performance and energy storage performance at a relatively high working temperature, and is suitable for high-temperature and high-operation field intensity working conditions.

1 . A biaxially oriented polypropylene dielectric film, characterized in that the raw material for preparing the biaxially oriented polypropylene dielectric film includes a modified polypropylene grafted with an alkenyl-containing functional monomer;
 the modified polypropylene grafted with an alkenyl-containing functional monomer comprises structural units derived from polypropylene as a matrix phase and structural units derived from the alkenyl-containing functional monomer as a dispersion phase; the modified polypropylene grafted with an alkenyl-containing functional monomer has an ash content of less than 50 ppm, preferably less than 36 ppm, and more preferably less than 30 ppm; the ratio of the mass of the structural units derived from the alkenyl-containing functional monomer and in a grafted state to the mass of the structural units derived from the alkenyl-containing functional monomer and in a self-polymerized state, in the modified polypropylene grafted with an alkenyl-containing functional monomer, is more than or equal to 1.0; the D50 of the dispersion phase is less than 450 nm, preferably 50-400 nm.   
     
     
         2 . The biaxially oriented polypropylene dielectric film according to  claim 1 , wherein the maximum working temperature of the biaxially oriented polypropylene dielectric film is ≥100° C., preferably 110-160° C., and more preferably 120-145° C. 
     
     
         3 . The biaxially oriented polypropylene dielectric film according to  claim 1 , wherein the biaxially oriented polypropylene dielectric film has at least one of the following characteristics:
 the breakdown field strength E g  at 120° C. is ≥500 MV/m, preferably 550-800 MV/m;   the direct current volume resistivity ρ vg  at 120° C. and 200 MV/m field strength is ≥6.0×10 13  Ω·m, preferably 1.0×10 14  Ω·m-1.0×10 20  Ω·m, preferably 1.5×10 14  Ω·m-0.9×10 20  Ω·m, more preferably 2.0×10 14  Ω·m-1.0×10 17  Ω·m;   the dielectric constant at 120° C. and 100 Hz is more than 2.25, preferably 2.26-2.65;   the dielectric loss at 120° C. and 100 Hz is less than 1.55E-3, preferably less than 1.5E-3, preferably less than or equal to 1.0E-3, more preferably 1.0E-6 to 1.3E-3, and further preferably 1.0E-6 to 9E-4;   the energy storage density at 120° C. and 300 MV/m is more than 0.720 J/cm 3  preferably 0.740-2.0 J/cm 3 , more preferably 0.780-2.0 J/cm 3 , and further preferably 0.80-2.0 J/cm 3 ;   the energy storage efficiency at 120° C. and 300 MV/m is more than 90.0%, and preferably 92.0-99.0%.   
     
     
         4 . The biaxially oriented polypropylene dielectric film according to  claim 1 , wherein the biaxially oriented polypropylene dielectric film has at least one of the following characteristics: the tensile strength in machine direction is ≥140 MPa, and preferably 140-170 MPa; the tensile strength in transverse direction is ≥200 MPa, and preferably 205-250 MPa; the elongation at break in machine direction is ≥210%, preferably ≥225%; the elongation at break in transverse direction is ≥60%, preferably ≥62%; the thickness is 0.5-15 μm, preferably 4-10 μm. 
     
     
         5 . The biaxially oriented polypropylene dielectric film according to  claim 1 , wherein the ratio of the mass of the structural units derived from the alkenyl-containing functional monomer and in a grafted state to the mass of the structural units derived from the alkenyl-containing functional monomer and in a self-polymerized state, in the modified polypropylene grafted with an alkenyl-containing functional monomer, is 1.1-1.0, preferably 1.2-6. 
     
     
         6 . The biaxially oriented polypropylene dielectric film according to  claim 1 , wherein the ratio of the mass of the structural units derived from the alkenyl-containing functional monomer and in a grafted state to the mass of the structural units derived from the alkenyl-containing functional monomer and in a self-polymerized state, in the modified polypropylene grafted with an alkenyl-containing functional monomer, is more than or equal to 1.5, preferably more than or equal to 2. 
     
     
         7 . The biaxially oriented polypropylene dielectric film according to  claim 1 , wherein the modified polypropylene grafted with an alkenyl-containing functional monomer has at least one of the following characteristics: the melt flow rate at the load of 2.16 kg at 230° C. is 1-10 g/10 min, preferably 1.5-8 g/10 min, and further preferably 2-5 g/10 min; the melting temperature T m  is 155-168° C., and preferably 157-165° C.; the flexural modulus is 1400-2000 MPa, preferably 1500-1800 MPa. 
     
     
         8 . The biaxially oriented polypropylene dielectric film according to  claim 1 , wherein the alkenyl-containing functional monomer is at least one selected from monomers having a structure represented by the formula 1, 
       
         
           
           
               
               
           
         
         in the formula 1, R b , R c , R d  are each independently selected from H, substituted or unsubstituted alkyl; R a  is selected from substituted or unsubstituted alkyl, substituted or unsubstituted alkoxy, substituted or unsubstituted aryl, substituted or unsubstituted ester group, substituted or unsubstituted carboxyl, substituted or unsubstituted cycloalkyl or heterocyclyl, cyano, substituted or unsubstituted silyl; R a  and R d  optionally form a ring. 
       
     
     
         9 . The biaxially oriented polypropylene dielectric film according to  claim 8 , wherein R b , R c , R d  are each independently selected from H, substituted or unsubstituted C1-C6 alkyl; R a  is selected from substituted or unsubstituted C1-C20 alkyl, substituted or unsubstituted C1-C20 alkoxy, substituted or unsubstituted C6-C20 aryl, substituted or unsubstituted C1-C20 ester group, substituted or unsubstituted C1-C20 carboxyl, substituted or unsubstituted C3-C20 cycloalkyl or heterocyclyl, cyano, substituted or unsubstituted C3-C20 silyl; the substituent group is halogen, —OH, —NH 2 , ═O, C1-C12 alkyl, C3-C6 cycloalkyl, C1-C12 alkoxy, C1-C12 acyloxy; R a  and R d  optionally taken together with the double bond form a 4-6 membered heterocyclic ring. 
     
     
         10 . The biaxially oriented polypropylene dielectric film according to  claim 9 , wherein R b , R c , R d  are each independently selected from H, substituted or unsubstituted C1-C6 alkyl;
 R a  is at least one selected from a group represented by formula 2, a group represented by formula 3, a group represented by formula 4, a group represented by formula 5, a group represented by formula 6 and a heterocyclic group;   
       
         
           
           
               
               
           
         
         in the formula 2, R 4 -R 8  are each independently selected from H, halogen, hydroxy, amino, phosphoric group, sulfonic group, substituted or unsubstituted C1-C12 alkyl, substituted or unsubstituted C3-C12 cycloalkyl, substituted or unsubstituted C1-C12 alkoxy, substituted or unsubstituted C1-C12 ester group, substituted or unsubstituted C1-C12 amine group, the substituent group is selected from halogen, hydroxy, amino, phosphoric group, sulfonic group, C1-C12 alkyl, C3-C12 cycloalkyl, C1-C12 alkoxy, C1-C12 ester group, C1-C12 amine group; preferably, R 4 -R 8  are each independently selected from H, halogen, hydroxy, amino, substituted or unsubstituted C1-C6 alkyl, substituted or unsubstituted C1-C6 alkoxy; 
       
       
         
           
           
               
               
           
         
         in the formula 3, R 4 -R 10  are each independently selected from H, halogen, hydroxy, amino, phosphoric group, sulfonic group, substituted or unsubstituted C1-C12 alkyl, substituted or unsubstituted C3-C12 cycloalkyl, substituted or unsubstituted C1-C12 alkoxy, substituted or unsubstituted C1-C12 ester group, substituted or unsubstituted C1-C12 amine group, the substituent group is selected from halogen, hydroxy, amino, phosphoric group, sulfonic group, C1-C12 alkyl, C3-C12 cycloalkyl, C1-C12 alkoxy, C1-C12 ester group, C1-C12 amine group; preferably, R 4 -R 10  are each independently selected from H, halogen, hydroxy, amino, substituted or unsubstituted C1-C6 alkyl, substituted or unsubstituted C1-C6 alkoxy, the substituent group is selected from halogen, hydroxy, amino, C1-C6 alkyl, C1-C6 alkoxy; 
       
       
         
           
           
               
               
           
         
         in the formula 4, R 4 ′-R 10 ′ are each independently selected from H, halogen, hydroxy, amino, phosphoric group, sulfonic group, substituted or unsubstituted C1-C12 alkyl, substituted or unsubstituted C3-C12 cycloalkyl, substituted or unsubstituted C1-C12 alkoxy, substituted or unsubstituted C1-C12 ester group, substituted or unsubstituted C1-C12 amine group, the substituent group is selected from halogen, hydroxy, amino, phosphoric group, sulfonic group, C1-C12 alkyl, C3-C12 cycloalkyl, C1-C12 alkoxy, C1-C12 ester group, C1-C12 amine group; preferably, R 4 ′-R 10 ′ are each independently selected from H, halogen, hydroxy, amino, substituted or unsubstituted C1-C6 alkyl, substituted or unsubstituted C1-C6 alkoxy, the substituent group is selected from halogen, hydroxy, amino, C1-C6 alkyl, C1-C6 alkoxy; 
       
       
         
           
           
               
               
           
         
         in the formula 5, R′, R″, R′″ are each independently selected from substituted or unsubstituted C1-C12 linear alkyl, substituted or unsubstituted C3-C12 branched alkyl, substituted or unsubstituted C1-C12 alkoxy, substituted or unsubstituted C1-C12 acyloxy; preferably, R 1  is C2-C6 alkenyl, preferably monounsaturated alkenyl; R 2 , R 3 , R 4  are each independently selected from substituted or unsubstituted C1-C6 linear alkyl, substituted or unsubstituted C3-C6 branched alkyl, substituted or unsubstituted C1-C6 alkoxy, substituted or unsubstituted C1-C6 acyloxy; 
       
       
         
           
           
               
               
           
         
         in the formula 6, R m  is selected from hydrogen and/or the following groups that are substituted or unsubstituted: C1-C20 linear alkyl, C3-C20 branched alkyl, C3-C12 cycloalkyl, C3-C12 epoxyalkyl, C3-C12 epoxyalkylalkyl, the substituent group is at least one selected from halogen, amino and hydroxy; 
         the heterocyclic group is selected from imidazolyl, pyrazolyl, carbazolyl, pyrrolidinonyl, pyridyl, piperidinyl, caprolactamyl, pyrazinyl, thiazolyl, purinyl, morpholinyl, oxazolinyl. 
       
     
     
         11 . The biaxially oriented polypropylene dielectric film according to  claim 10 , wherein the alkenyl-containing functional monomer is a styrenic monomer, the styrenic monomer is at least one selected from styrene, α-methylstyrene, 1-vinyl naphthalene, 2-vinyl naphthalene, mono- or polysubstituted styrene, mono- or polysubstituted α-methylstyrene, mono- or polysubstituted 1-vinyl naphthalene and mono- or polysubstituted 2-vinyl naphthalene; the substituent group preferably is at least one selected from halogen, hydroxy, amino, phosphoric group, sulfonic group, C1-C8 linear alkyl, C3-C8 branched alkyl or cycloalkyl, C1-C6 linear alkoxy, C3-C8 branched alkoxy or cycloalkoxy, C1-C8 linear ester group, C3-C8 branched ester group or cyclic ester group, C1-C8 linear amine group and C3-C8 branched amine group or cyclic amine group; preferably, the styrenic monomer is at least one selected from styrene, α-methylstyrene, 2-methylstyrene, 3-methylstyrene and 4-methylstyrene. 
     
     
         12 . The biaxially oriented polypropylene dielectric film according to  claim 10 , wherein the alkenyl-containing functional monomer is an alkenyl-containing silane monomer, the alkenyl-containing silane monomer is at least one selected from vinyltriethoxysilane, vinyltrimethoxysilane, vinyltriisopropoxysilane, vinyltri-tert-butoxysilane, vinyltriacetoxysilane, methylvinyldimethoxysilane, ethylvinyldiethoxysilane, allyltriethoxysilane, allyltrimethoxysilane, allyltriisopropoxysilane, vinyltris (P-methoxyethoxy)silane, allyltris (P-methoxyethoxy)silane, allyltri-tert-butoxysilane, allyltriacetoxysilane, methylallyldimethoxysilane and ethylallyldiethoxysilane. 
     
     
         13 . The biaxially oriented polypropylene dielectric film according to  claim 10 , wherein the alkenyl-containing functional monomer is an acrylate monomer and/or an acrylic monomer, preferably, the acrylate monomer is at least one selected from methyl (methyl)acrylate, sec-butyl (methyl)acrylate, ethyl (methyl)acrylate, n-butyl (methyl)acrylate, isobutyl (methyl)acrylate, tert-butyl (methyl)acrylate, isooctyl (methyl)acrylate, dodecyl (methyl)acrylate, cocinin (methyl)acrylate, octadecyl (methyl)acrylate, dimethylaminoethyl (methyl)acrylate, diethylaminoethyl (methyl)acrylate, dimethylaminopropyl (methyl)acrylate, and glycidyl (methyl)acrylate; preferably, the acrylic monomer is at least one selected from acrylic acid, methacrylic acid and 2-ethylacrylic acid. 
     
     
         14 . The biaxially oriented polypropylene dielectric film according to  claim 9 , wherein the alkenyl-containing functional monomer is at least one selected from maleic anhydride, maleimide and derivatives thereof, itaconic anhydride, and α-methylene-γ-butyrolactone; preferably, the alkenyl-containing functional monomer is maleic anhydride. 
     
     
         15 . The biaxially oriented polypropylene dielectric film according to  claim 1 , wherein the polypropylene has the following characteristics: the particle size is 16-50 meshes; the ash content is less than 55 ppm, preferably less than 40 ppm, more preferably less than 35 ppm; the flexural modulus is 1400-2000 MPa, preferably 1500-1800 MPa. 
     
     
         16 . The biaxially oriented polypropylene dielectric film according to  claim 1 , wherein the polypropylene is a homopolypropylene or a propylene copolymer, and preferably has at least one of the following characteristics:
 the melt flow rate at the load of 2.16 kg at 230° C. is 0.5-10 g/10 min, preferably 1-5 g/10 min, and further preferably 2-4 g/10 min;   the melting temperature T m  is 150° C. or higher, preferably 153-180° C., further preferably 155-167° C.;   the isotacticity is more than 96%, preferably more than 96.5%; or, the total content of ethylene units and butene units is less than 3.0 mol %, preferably more than 0 and less than 0.1 mol %, or more than 0.1 mol % and less than or equal to 3.0 mol %.   
     
     
         17 . The biaxially oriented polypropylene dielectric film according to  claim 1 , wherein the modified polypropylene grafted with an alkenyl-containing functional monomer is prepared by a method comprising the following steps: in the presence of an inert gas, a reaction mixture comprising a polypropylene powder and an alkenyl-containing functional monomer is subjected to grafting reaction, and is optionally washed by a washing solvent, to obtain the modified polypropylene grafted with an alkenyl-containing functional monomer. 
     
     
         18 . The biaxially oriented polypropylene dielectric film according to  claim 17 , wherein the reaction mixture further comprises a free radical initiator and at least one of the following components: deionized water and/or an organic solvent, wherein the mass content of the deionized water is 300-800% of the sum of the mass of the polypropylene powder and the alkenyl-containing functional monomer, and the mass content of the organic solvent is 1-35% of the mass of the polypropylene powder. 
     
     
         19 . The biaxially oriented polypropylene dielectric film according to  claim 18 , wherein the preparation method of the modified polypropylene grafted with an alkenyl-containing functional monomer comprises the following steps:
 a. placing a polypropylene powder in a closed reactor, followed by inert gas replacement;   b. adding a free radical initiator and an alkenyl-containing functional monomer to the closed reactor, and mixing with stirring;   c. adding deionized water, heating the reaction system to the grafting reaction temperature, to carry out the grafting reaction; optionally swelling the reaction system before or after adding deionized water;   d. after the end of the reaction, filtering and optionally drying to obtain a powder;   e. optionally, washing the powder by a washing solvent, filtering and drying;   f. obtaining the modified polypropylene grafted with an alkenyl-containing functional monomer; or   the preparation method of the modified polypropylene grafted with an alkenyl-containing functional monomer comprises the following steps:   a. placing a polypropylene powder in a closed reactor, followed by inert gas replacement;   b. mixing an organic solvent and a free radical initiator, and adding the mixture to the closed reactor;   c. removing the organic solvent, adding an alkenyl-containing functional monomer, and optionally swelling the reaction system;   d. adding deionized water, heating the reaction system to the grafting reaction temperature, to carry out the grafting reaction;   e. after the end of the reaction, filtering and optionally drying to obtain a powder;   f. optionally, washing the powder by a washing solvent, filtering and drying;   g. obtaining the modified polypropylene grafted with an alkenyl-containing functional monomer;   wherein, the temperature of the grafting reaction is 30-110° C., and preferably 60-95° C.; the time is 0.5-10 h, and preferably 1-6 h.   
     
     
         20 . The biaxially oriented polypropylene dielectric film according to  claim 17 , wherein the method further comprises a step of screening pretreatment of the polypropylene powder, preferably, the screening pretreatment comprises the steps of screening the powder by using a double-layer vibrating screen or a linear screen with a screen mesh of a corresponding mesh number, and using the polypropylene powder in the middle layer of the screen as a reaction material. 
     
     
         21 . The biaxially oriented polypropylene dielectric film according to  claim 17 , wherein the washing solvent is one or a mixture of two or more selected from the group consisting of n-hexane, cyclohexane, n-heptane, petroleum ether, ethyl acetate, propyl acetate, isopropyl acetate, butyl acetate, acetone, methyl ethyl ketone, tetrahydrofuran, 2-methyl tetrahydrofuran, benzene, toluene and xylene; preferably, the washing solvent is a mixture of solvent A and solvent B, wherein the solvent A is at least one selected from the group consisting of n-hexane, cyclohexane, n-heptane, petroleum ether, ethyl acetate, propyl acetate, isopropyl acetate, butyl acetate, acetone, methyl ethyl ketone, tetrahydrofuran, and 2-methyl tetrahydrofuran, and the solvent B is at least one selected from the group consisting of toluene and xylene;
 the mass of the washing solvent is 2-20 times of the mass of the grafted polypropylene powder.   
     
     
         22 . The biaxially oriented polypropylene dielectric film according to  claim 17 , wherein the washing temperature is 40-130° C., preferably 55-100° C., and the washing time is 0.5-10 h, preferably 1-8 h. 
     
     
         23 . The biaxially oriented polypropylene dielectric film according to  claim 1 , wherein the raw material for preparing the biaxially oriented polypropylene dielectric film further comprises an antioxidant and optionally a processing aid, wherein the content of the antioxidant is 0.1-0.8 part by weight, preferably 0.1-0.6 part by weight, and the content of the processing aid is 0.05-1 part by weight, preferably 0.05-0.5 part by weight, based on 100 parts by weight of the modified polypropylene; the content of the modified polypropylene grafted with an alkenyl-containing functional monomer is 50 wt % or more, preferably 60 wt % or more, more preferably 70 wt % or more, based on the weight of the dielectric film. 
     
     
         24 . The biaxially oriented polypropylene dielectric film according to  claim 23 , wherein the antioxidant is one or a mixture of two or more selected from the group consisting of hindered phenols, hindered amines, phosphites, thios, benzofuranones;
 preferably, the antioxidant is obtained by compounding an antioxidant component A and an antioxidant component B, wherein the antioxidant component A is at least one selected from the group consisting of hindered phenols, hindered amines, phosphites and thios, and the antioxidant component B is at least one benzofuranone;   more preferably, the antioxidant component A is at least one selected from the group consisting of pentaerythrityl tetrakis[beta-(3,5-di-t-butyl-4-hydroxyphenyl) propionate], tris[2,4-di-t-butylphenyl]phosphite, n-octadecyl beta-(3,5-di-t-butyl-4-hydroxyphenyl) propionate, 2,2′-methylene bis(4-methyl-6-t-butylphenol), 2,2′-thiobis[3-(3,5-di-t-butyl-4-hydroxyphenyl) propionate], 1,1,3-tris (2-methyl-4-hydroxy-5-t-butylphenyl) butane, and bis(2,4-di-tert-butylphenol) pentaerythritol diphosphite; the antioxidant component B is at least one selected from the group consisting of 3-butyl-1-(3-hydrogen)-isobenzofuranone, 5-methyl-7-tert-butyl-3-(3,4-dimethyl)-3-hydrogen-benzofuran-2-one, 5-methyl-7-tert-butyl-3-(2,5-dimethyl)-3-hydrogen-benzofuran-2-one and 5-methyl-7-tert-butyl-3-(2-hydroxy-5-methyl)-3-hydrogen-benzofuran-2-one;   the weight ratio of the antioxidant component A to the antioxidant component B is 1:0.01-0.03.   
     
     
         25 . The biaxially oriented polypropylene dielectric film according to  claim 23 , wherein the processing aid is at least one selected from the group consisting of a lubricant, an acid acceptor, a slip agent, an antistatic agent and an antiblock agent;
 the lubricant is at least one selected from the group consisting of polyethylene glycol lubricants, fluoropolymer lubricants, organosilicon lubricants, fatty alcohol lubricants, fatty acid lubricants, fatty acid ester lubricants, stearic acid amide lubricants, fatty acid metal soap lubricants, alkane and oxidized alkane lubricants and micro-nano particle lubricants.   
     
     
         26 . The biaxially oriented polypropylene dielectric film according to  claim 1 , wherein the biaxially oriented polypropylene dielectric film is prepared by a method comprising the steps of:
 (1) mixing and pelletizing raw materials for the preparation;   (2) melt extruding the pellets obtained in step (1) and casting, to obtain a modified polypropylene cast sheet;   (3) biaxially stretching the modified polypropylene cast sheet to obtain the biaxially oriented polypropylene dielectric film.   
     
     
         27 . A modified polypropylene material for preparing dielectric films, characterized in that: the modified polypropylene material comprises a modified polypropylene grafted with an alkenyl-containing functional monomer, wherein the modified polypropylene grafted with an alkenyl-containing functional monomer comprises structural units derived from polypropylene as a matrix phase and structural units derived from the alkenyl-containing functional monomer as a dispersion phase; the modified polypropylene grafted with an alkenyl-containing functional monomer has an ash content of less than 50 ppm, preferably less than 36 ppm, and more preferably less than 30 ppm; the ratio of the mass of the structural units derived from the alkenyl-containing functional monomer and in a grafted state to the mass of the structural units derived from the alkenyl-containing functional monomer and in a self-polymerized state, in the modified polypropylene grafted with an alkenyl-containing functional monomer, is more than or equal to 1.0; the D50 of the dispersion phase is less than 450 nm, preferably 50-400 nm. 
     
     
         28 . The modified polypropylene material according to  claim 27 , wherein the maximum working temperature of the modified polypropylene material is ≥100° C., preferably 110-160° C., and more preferably 120-145° C. 
     
     
         29 . The modified polypropylene material according to  claim 27 , wherein the modified polypropylene material has at least one of the following characteristics:
 the breakdown field strength E g  at 120° C. is ≥500 MV/m, preferably 550-800 MV/m;   the direct current volume resistivity ρ vg  at 120° C. and 200 MV/m field strength is ≥6.0×10 13  Ω·m, preferably 1.0×10 14  Ω·m-1.0×10 20  Ω·m, preferably 1.5×10 14  Ω·m-0.9×10 20  Ω·m, more preferably 2.0×10 14  Ω·m-1.0×10 17  Ω·m;   the dielectric constant at 120° C. and 100 Hz is more than 2.25, preferably 2.26-2.65;   the dielectric loss at 120° C. and 100 Hz is less than 1.55E-3, preferably less than 1.5E-3, preferably less than or equal to 1.0E-3, more preferably 1.0E-6 to 1.3E-3, and further preferably 1.0E-6 to 9E-4;   the energy storage density at 120° C. and 300 MV/m is more than 0.720 J/cm 3  preferably 0.740-2.0 J/cm 3 , more preferably 0.780-2.0 J/cm 3 , and further preferably 0.80-2.0 J/cm 3      the energy storage efficiency at 120° C. and 300 MV/m is more than 90.0%, and preferably 92.0-99.0%.   
     
     
         30 . The modified polypropylene material according to  claim 27 , wherein the ratio of the mass of the structural units derived from the alkenyl-containing functional monomer and in a grafted state to the mass of the structural units derived from the alkenyl-containing functional monomer and in a self-polymerized state, in the modified polypropylene grafted with an alkenyl-containing functional monomer, is 1.1-1.0, preferably 1.2-6. 
     
     
         31 . The modified polypropylene material according to  claim 27 , wherein the ratio of the mass of the structural units derived from the alkenyl-containing functional monomer and in a grafted state to the mass of the structural units derived from the alkenyl-containing functional monomer and in a self-polymerized state, in the modified polypropylene grafted with an alkenyl-containing functional monomer, is more than or equal to 1.5, preferably more than or equal to 2. 
     
     
         32 . The modified polypropylene material according to  claim 27 , wherein the modified polypropylene grafted with an alkenyl-containing functional monomer has at least one of the following characteristics: the melt flow rate at the load of 2.16 kg at 230° C. is 1-10 g/10 min, preferably 1.5-8 g/10 min, and further preferably 2-5 g/10 min; the melting temperature T m  is 155-168° C., and preferably 157-165° C.; the flexural modulus is 1400-2000 MPa, preferably 1500-1800 MPa. 
     
     
         33 . The modified polypropylene material according to  claim 27 , wherein the alkenyl-containing functional monomer is at least one selected from monomers having a structure represented by the formula 1, 
       
         
           
           
               
               
           
         
         in the formula 1, R b , R c , R d  are each independently selected from H, substituted or unsubstituted alkyl; R a  is selected from substituted or unsubstituted alkyl, substituted or unsubstituted alkoxy, substituted or unsubstituted aryl, substituted or unsubstituted ester group, substituted or unsubstituted carboxyl, substituted or unsubstituted cycloalkyl or heterocyclyl, cyano, substituted or unsubstituted silyl; R a  and R d  optionally form a ring. 
       
     
     
         34 . The modified polypropylene material according to  claim 33 , wherein R b , R c , R d  are each independently selected from H, substituted or unsubstituted C1-C6 alkyl; R a  is selected from substituted or unsubstituted C1-C20 alkyl, substituted or unsubstituted C1-C20 alkoxy, substituted or unsubstituted C6-C20 aryl, substituted or unsubstituted C1-C20 ester group, substituted or unsubstituted C1-C20 carboxyl, substituted or unsubstituted C3-C20 cycloalkyl or heterocyclyl, cyano, substituted or unsubstituted C3-C20 silyl; the substituent group is halogen, —OH, —NH 2 , ═O, C 1 -C 12  alkyl, C 3 -C 6  cycloalkyl, C 1 -C 12  alkoxy, C 1 -C 12  acyloxy; R a  and R d  optionally taken together with the double bond form a 4-6 membered heterocyclic ring. 
     
     
         35 . The modified polypropylene material according to  claim 34 , wherein R b , R c , R d  are each independently selected from H, substituted or unsubstituted C 1 -C 6  alkyl;
 R a  is at least one selected from a group represented by formula 2, a group represented by formula 3, a group represented by formula 4, a group represented by formula 5, a group represented by formula 6 and a heterocyclic group;   
       
         
           
           
               
               
           
         
         in the formula 2, R 4 -R 8  are each independently selected from H, halogen, hydroxy, amino, phosphoric group, sulfonic group, substituted or unsubstituted C 1 -C 12  alkyl, substituted or unsubstituted C 3 -C 12  cycloalkyl, substituted or unsubstituted C 1 -C 12  alkoxy, substituted or unsubstituted C 1 -C 12  ester group, substituted or unsubstituted C 1 -C 12  amine group, the substituent group is selected from halogen, hydroxy, amino, phosphoric group, sulfonic group, C 1 -C 12  alkyl, C 3 -C 12  cycloalkyl, C 1 -C 12  alkoxy, C 1 -C 12  ester group, C 1 -C 12  amine group; preferably, R 4 -R 8  are each independently selected from H, halogen, hydroxy, amino, substituted or unsubstituted C 1 -C 6  alkyl, substituted or unsubstituted C 1 -C 6  alkoxy; 
       
       
         
           
           
               
               
           
         
         in the formula 3, R 4 -R 10  are each independently selected from H, halogen, hydroxy, amino, phosphoric group, sulfonic group, substituted or unsubstituted C 1 -C 12  alkyl, substituted or unsubstituted C 3 -C 12  cycloalkyl, substituted or unsubstituted C 1 -C 12  alkoxy, substituted or unsubstituted C 1 -C 12  ester group, substituted or unsubstituted C 1 -C 12  amine group, the substituent group is selected from halogen, hydroxy, amino, phosphoric group, sulfonic group, C 1 -C 12  alkyl, C 3 -C 12  cycloalkyl, C 1 -C 12  alkoxy, C 1 -C 12  ester group, C 1 -C 12  amine group; preferably, R 4 -R 10  are each independently selected from H, halogen, hydroxy, amino, substituted or unsubstituted C 1 -C 6  alkyl, substituted or unsubstituted C 1 -C 6  alkoxy, the substituent group is selected from halogen, hydroxy, amino, C 1 -C 6  alkyl, C 1 -C 6  alkoxy; 
       
       
         
           
           
               
               
           
         
         in the formula 4, R 4 ′-R 10 ′ are each independently selected from H, halogen, hydroxy, amino, phosphoric group, sulfonic group, substituted or unsubstituted C 1 -C 12  alkyl, substituted or unsubstituted C 3 -C 12  cycloalkyl, substituted or unsubstituted C 1 -C 12  alkoxy, substituted or unsubstituted C 1 -C 12  ester group, substituted or unsubstituted C 1 -C 12  amine group, the substituent group is selected from halogen, hydroxy, amino, phosphoric group, sulfonic group, C 1 -C 12  alkyl, C 3 -C 12  cycloalkyl, C 1 -C 12  alkoxy, C 1 -C 12  ester group, C 1 -C 12  amine group; preferably, R 4 ′-R 10 ′ are each independently selected from H, halogen, hydroxy, amino, substituted or unsubstituted C 1 -C 6  alkyl, substituted or unsubstituted C 1 -C 6  alkoxy, the substituent group is selected from halogen, hydroxy, amino, C 1 -C 6  alkyl, C 1 -C 6  alkoxy; 
       
       
         
           
           
               
               
           
         
         in the formula 5, R′, R″, R′″ are each independently selected from substituted or unsubstituted C 1 -C 12  linear alkyl, substituted or unsubstituted C 3 -C 12  branched alkyl, substituted or unsubstituted C 1 -C 12  alkoxy, substituted or unsubstituted C 1 -C 12  acyloxy; preferably, R 1  is C 2 -C 6  alkenyl, preferably monounsaturated alkenyl; R 2 , R 3 , R 4  are each independently selected from substituted or unsubstituted C 1 -C 6  linear alkyl, substituted or unsubstituted C 3 -C 6  branched alkyl, substituted or unsubstituted C 1 -C 6  alkoxy, substituted or unsubstituted C 1 -C 6  acyloxy; 
       
       
         
           
           
               
               
           
         
         in the formula 6, R m  is selected from hydrogen and/or the following groups that are substituted or unsubstituted: C 1 -C 20  linear alkyl, C 3 -C 20  branched alkyl, C 3 -C 12  cycloalkyl, C 3 -C 12  epoxyalkyl, C 3 -C 12  epoxyalkylalkyl, the substituent group is at least one selected from halogen, amino and hydroxy; 
         the heterocyclic group is selected from imidazolyl, pyrazolyl, carbazolyl, pyrrolidinonyl, pyridyl, piperidinyl, caprolactamyl, pyrazinyl, thiazolyl, purinyl, morpholinyl, oxazolinyl. 
       
     
     
         36 . The modified polypropylene material according to  claim 35 , wherein the alkenyl-containing functional monomer is a styrenic monomer, the styrenic monomer is at least one selected from styrene, α-methylstyrene, 1-vinyl naphthalene, 2-vinyl naphthalene, mono- or polysubstituted styrene, mono- or polysubstituted α-methylstyrene, mono- or polysubstituted 1-vinyl naphthalene and mono- or polysubstituted 2-vinyl naphthalene; the substituent group preferably is at least one selected from halogen, hydroxy, amino, phosphoric group, sulfonic group, C 1 -C 8  linear alkyl, C 3 -C 8  branched alkyl or cycloalkyl, C 1 -C 6  linear alkoxy, C 3 -C 8  branched alkoxy or cycloalkoxy, C 1 -C 8  linear ester group, C 3 -C 8  branched ester group or cyclic ester group, C 1 -C 8  linear amine group and C 3 -C 8  branched amine group or cyclic amine group; preferably, the styrenic monomer is at least one selected from styrene, α-methylstyrene, 2-methylstyrene, 3-methylstyrene and 4-methylstyrene. 
     
     
         37 . The modified polypropylene material according to  claim 35 , wherein the alkenyl-containing functional monomer is an alkenyl-containing silane monomer, the alkenyl-containing silane monomer is at least one selected from vinyltriethoxysilane, vinyltrimethoxysilane, vinyltriisopropoxysilane, vinyltri-tert-butoxysilane, vinyltriacetoxysilane, methylvinyldimethoxysilane, ethylvinyldiethoxysilane, allyltriethoxysilane, allyltrimethoxysilane, allyltriisopropoxysilane, vinyltris (P-methoxyethoxy)silane, allyltris (P-methoxyethoxy)silane, allyltri-tert-butoxysilane, allyltriacetoxysilane, methylallyldimethoxysilane and ethylallyldiethoxysilane. 
     
     
         38 . The modified polypropylene material according to  claim 35 , wherein the alkenyl-containing functional monomer is an acrylate monomer and/or an acrylic monomer, preferably, the acrylate monomer is at least one selected from methyl (methyl)acrylate, sec-butyl (methyl)acrylate, ethyl (methyl)acrylate, n-butyl (methyl)acrylate, isobutyl (methyl)acrylate, tert-butyl (methyl)acrylate, isooctyl (methyl)acrylate, dodecyl (methyl)acrylate, cocinin (methyl)acrylate, octadecyl (methyl)acrylate, dimethylaminoethyl (methyl)acrylate, diethylaminoethyl (methyl)acrylate, dimethylaminopropyl (methyl)acrylate, and glycidyl (methyl)acrylate; preferably, the acrylic monomer is at least one selected from acrylic acid, methacrylic acid and 2-ethylacrylic acid. 
     
     
         39 . The modified polypropylene material according to  claim 34 , wherein the alkenyl-containing functional monomer is at least one selected from maleic anhydride, maleimide and derivatives thereof, itaconic anhydride, and α-methylene-γ-butyrolactone; preferably, the alkenyl-containing functional monomer is maleic anhydride. 
     
     
         40 . The modified polypropylene material according to  claim 27 , wherein the polypropylene has the following characteristics: the particle size is 16-50 meshes; the ash content is less than 55 ppm, preferably less than 40 ppm, more preferably less than 35 ppm; the flexural modulus is 1400-2000 MPa, preferably 1500-1800 MPa. 
     
     
         41 . The modified polypropylene material according to  claim 27 , wherein the polypropylene is a homopolypropylene or a propylene copolymer, and preferably has at least one of the following characteristics:
 the melt flow rate at the load of 2.16 kg at 230° C. is 0.5-10 g/10 min, preferably 1-5 g/10 min, and further preferably 2-4 g/10 min;   the melting temperature T m  is 150° C. or higher, preferably 153-180° C., further preferably 155-167° C.;   the isotacticity is more than 96%, preferably more than 96.5%; or, the total content of ethylene units and butene units is less than 3.0 mol %, preferably more than 0 and less than 0.1 mol %, or more than 0.1 mol % and less than or equal to 3.0 mol %.   
     
     
         42 . The modified polypropylene material according to  claim 27 , wherein the modified polypropylene grafted with an alkenyl-containing functional monomer is prepared by a method comprising the following steps: in the presence of an inert gas, a reaction mixture comprising a polypropylene powder and an alkenyl-containing functional monomer is subjected to grafting reaction, and is optionally washed by a washing solvent, to obtain the modified polypropylene grafted with an alkenyl-containing functional monomer. 
     
     
         43 . The modified polypropylene material according to  claim 42 , wherein the reaction mixture further comprises a free radical initiator and at least one of the following components: deionized water and/or an organic solvent, wherein the mass content of the deionized water is 300-800% of the sum of the mass of the polypropylene powder and the alkenyl-containing functional monomer, and the mass content of the organic solvent is 1-35% of the mass of the polypropylene powder. 
     
     
         44 . The modified polypropylene material according to  claim 43 , wherein the preparation method of the modified polypropylene grafted with an alkenyl-containing functional monomer comprises the following steps:
 a. placing a polypropylene powder in a closed reactor, followed by inert gas replacement;   b. adding a free radical initiator and an alkenyl-containing functional monomer to the closed reactor, and mixing with stirring;   c. adding deionized water, heating the reaction system to the grafting reaction temperature, to carry out the grafting reaction; optionally swelling the reaction system before or after adding deionized water;   d. after the end of the reaction, filtering and optionally drying to obtain a powder;   e. optionally, washing the powder by a washing solvent, filtering and drying;   f. obtaining the modified polypropylene grafted with an alkenyl-containing functional monomer; or   the preparation method of the modified polypropylene grafted with an alkenyl-containing functional monomer comprises the following steps:   a. placing a polypropylene powder in a closed reactor, followed by inert gas replacement;   b. mixing an organic solvent and a free radical initiator, and adding the mixture to the closed reactor;   c. removing the organic solvent, adding an alkenyl-containing functional monomer, and optionally swelling the reaction system;   d. adding deionized water, heating the reaction system to the grafting reaction temperature, to carry out the grafting reaction;   e. after the end of the reaction, filtering and optionally drying to obtain a powder;   f. optionally, washing the powder by a washing solvent, filtering and drying;   g. obtaining the modified polypropylene grafted with an alkenyl-containing functional monomer;   wherein, the temperature of the grafting reaction is 30-110° C., and preferably 60-95° C.; the time is 0.5-10 h, and preferably 1-6 h.   
     
     
         45 . The modified polypropylene material according to  claim 42 , wherein the method further comprises a step of screening pretreatment of the polypropylene powder, preferably, the screening pretreatment comprises the steps of screening the powder by using a double-layer vibrating screen or a linear screen with a screen mesh of a corresponding mesh number, and using the polypropylene powder in the middle layer of the screen as a reaction material. 
     
     
         46 . The modified polypropylene material according to  claim 42 , wherein the washing solvent is one or a mixture of two or more selected from the group consisting of n-hexane, cyclohexane, n-heptane, petroleum ether, ethyl acetate, propyl acetate, isopropyl acetate, butyl acetate, acetone, methyl ethyl ketone, tetrahydrofuran, 2-methyl tetrahydrofuran, benzene, toluene and xylene; preferably, the washing solvent is a mixture of solvent A and solvent B, wherein the solvent A is at least one selected from the group consisting of n-hexane, cyclohexane, n-heptane, petroleum ether, ethyl acetate, propyl acetate, isopropyl acetate, butyl acetate, acetone, methyl ethyl ketone, tetrahydrofuran, and 2-methyl tetrahydrofuran, and the solvent B is at least one selected from the group consisting of toluene and xylene;
 the mass of the washing solvent is 2-20 times of the mass of the grafted polypropylene powder.   
     
     
         47 . The modified polypropylene material according to in  claim 42 , wherein the washing temperature is 40-130° C., preferably 55-100° C., and the washing time is 0.5-10 h, preferably 1-8 h. 
     
     
         48 . The modified polypropylene material according to  claim 27 , characterized in that the modified polypropylene material also comprise an antioxidant and optionally a processing aid, wherein the content of the antioxidant is 0.1-0.8 part by weight, preferably 0.1-0.6 part by weight, and the content of the processing aid is 0.05-1 part by weight, preferably 0.05-0.5 part by weight, based on 100 parts by weight of the modified polypropylene. 
     
     
         49 . The modified polypropylene material according to  claim 48 , wherein the antioxidant is one or a mixture of two or more selected from the group consisting of hindered phenols, hindered amines, phosphites, thios, benzofuranones;
 preferably, the antioxidant is obtained by compounding an antioxidant component A and an antioxidant component B, wherein the antioxidant component A is at least one selected from the group consisting of hindered phenols, hindered amines, phosphites and thios, and the antioxidant component B is at least one benzofuranone;   more preferably, the antioxidant component A is at least one selected from the group consisting of pentaerythrityl tetrakis[beta-(3,5-di-t-butyl-4-hydroxyphenyl) propionate], tris[2,4-di-t-butylphenyl]phosphite, n-octadecyl beta-(3,5-di-t-butyl-4-hydroxyphenyl) propionate, 2,2′-methylene bis(4-methyl-6-t-butylphenol), 2,2′-thiobis[3-(3,5-di-t-butyl-4-hydroxyphenyl) propionate], 1,1,3-tris (2-methyl-4-hydroxy-5-t-butylphenyl) butane, and bis(2,4-di-tert-butylphenol) pentaerythritol diphosphite; the antioxidant component B is at least one selected from the group consisting of 3-butyl-1-(3-hydrogen)-isobenzofuranone, 5-methyl-7-tert-butyl-3-(3,4-dimethyl)-3-hydrogen-benzofuran-2-one, 5-methyl-7-tert-butyl-3-(2,5-dimethyl)-3-hydrogen-benzofuran-2-one and 5-methyl-7-tert-butyl-3-(2-hydroxy-5-methyl)-3-hydrogen-benzofuran-2-one;   the weight ratio of the antioxidant component A to the antioxidant component B is 1:0.01-0.03.   
     
     
         50 . The modified polypropylene material according to  claim 48 , wherein the processing aid is at least one selected from the group consisting of a lubricant, an acid acceptor, a slip agent, an antistatic agent and an antiblock agent;
 the lubricant is at least one selected from the group consisting of polyethylene glycol lubricants, fluoropolymer lubricants, organosilicon lubricants, fatty alcohol lubricants, fatty acid lubricants, fatty acid ester lubricants, stearic acid amide lubricants, fatty acid metal soap lubricants, alkane and oxidized alkane lubricants and micro-nano particle lubricants.   
     
     
         51 . An energy storage dielectric medium comprising the biaxially oriented polypropylene dielectric film according to  claim 1 , in particular a high temperature energy storage dielectric medium, wherein the dielectric medium is preferably a monolayer/multilayer dielectric film, preferably a capacitor film, an electrical film, a rough film, a supercapacitor film, an electrostatic film, or a battery separator film. 
     
     
         52 . A polypropylene capacitor film, being a monolayer or a multilayer, wherein at least one layer is the biaxially oriented polypropylene dielectric film according to  claim 1 . 
     
     
         53 . A polypropylene electrical film, being a monolayer or a multilayer, wherein at least one layer is the biaxially oriented polypropylene dielectric film according to  claim 1 . 
     
     
         54 . An energy storage dielectric medium comprising the modified polypropylene material according to  claim 27 , in particular a high temperature energy storage dielectric medium, wherein the dielectric medium is preferably a monolayer/multilayer dielectric film, preferably a capacitor film, an electrical film, a rough film, a supercapacitor film, an electrostatic film, or a battery separator film. 
     
     
         55 . A polypropylene capacitor film, being a monolayer or a multilayer, wherein at least one layer is made of raw materials containing the modified polypropylene material according to  claim 27 , preferably by biaxial stretching, preferably wherein at least one layer is made of the modified polypropylene material according to  claim 27  by biaxial stretching. 
     
     
         56 . A polypropylene electrical film, being a monolayer or a multilayer, wherein at least one layer is made of raw materials containing the modified polypropylene material according to  claim 27 , preferably by biaxial stretching, preferably wherein at least one layer is made of the modified polypropylene material according to  claim 27  by biaxial stretching.